1. Field of the Invention
This invention relates to a multi-gap type spark plug in which a plurality of outer electrodes are arranged opposed to a center electrode, and particularly concerns a multi-gap type spark plug directed to an improvement of a gap relationship among the electrodes.
2. Description of Prior Art
In a multi-gap type spark plug in which an insulator and a center electrode are in turn enclosed into a metallic shell, three outer electrodes are provided in opposition to a center electrode as shown in Japanese Patent Provisional Publication Nos. 51-95540 and 53-95443. In the former reference a main gap is dimensionally determined to be less than a summation of a secondary gap and a surface creeping gap so as to improve an ignition against a lean fuel gas mixture. In the latter reference, a first spark gap is dimensionally determined to be greater than a second spark gap, so that a voltage needed to discharge at the first spark gap is greater than that of the second spark gap.
Both the references, are directed to an ignition performance which tends to be inferior in comparison to a single gap-type spark plug because the outer electrodes decrease the chances of allowing the fuel gas mixture to pass through the spark gap when the fuel gas mixture is introduced into an engine cylinder.
In order to improve the ignition performance from, it has been resorted to adjusting a length dimension in which a front end of a leg portion of the insulator extends beyond that of the metallic shell. The leg portion of the insulator is a lower half portion which is tapered toward a front end thereof. It has been required to shorten the leg portion by 0.5 mm to 2.0 mm so as to ensure a heat-resistant property comparable to that of an ordinary spark plug which has a L-shaped outer electrode can achieve.
As the front end of the insulator extends beyond that of the metallic shell, a distance between the front end of the insulator and the outer electrode is shortened so as to cause semi-creeping discharge or channeling although the extended front end of the insulator is effectively cooled by the intake fuel gas mixture.
On the other hand, as an entire length of the leg portion is shortened to dimensionally decrease the front end which the leg portion extends beyond the metallic shell, a discharge spark between the electrodes decreases chances to run along a fouled surface of the front end of the insulator so as to hinder self-cleaning action although decreased heat capacity of the leg portion improves its heat dissipation.
Nowadays, it is common to dimensionally decrease the front end which the leg portion extends beyond the metallic shell with the self-cleaning action somewhat sacrificed, so that the front end of the leg portion is vulnerable to fouling due to a deposit of carbon particles produced when the fuel gas mixture is burned at the time of an ignition.
Therefore, it is an object of the invention to eliminate the above drawbacks on the basis that a minimum distance between the outer electrode and a front surface end of the insulator is found not to be so strictly necessary. The invention provides a multi-gap type spark plug which enables lengthening the front end of the leg portion without diminishing the leg portion to favorably dissipate heat from the leg portion, and at the same time achieving an improved self-cleaning action so as to protect the front end of the leg portion against fouling.